It has been hoped to obtain an amplifier circuit having a good linear response characteristic and a good linear impedance-varying characteristic. However, it has been found that the linearities of the response and impedance-varying characteristics of conventional circuits using transistors are insufficient because conventional transistors exhibit non-linear mutual conductance characteristics varying with input voltage.
Therefore, conventional transistor amplifier circuits require negative feedback for restraining their non-linearities. But negative feedback decreases the gain of an amplifier circuit. In the past, many efforts have been made for improving the linearity of amplifier circuits.